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Suppressed Subsonic Sound Levels

This post follows the introduction to shots, pops, and sound pressure levels. Virtually all firearms create pressure levels above 140dB, which is the limit established in MIL-STD-1474D to avoid unacceptable hearing damage. Hence, we put suppressors on our guns to bring their peak noise down to “hearing-safe” levels.

Small subsonic loads, like a 40gr .22″ bullet leaving a suppressed rifle muzzle at just 1000fps, make peak sound levels that are roughly the same as manually cycling the bolt of the gun shooting them: about 120dB. (Without a suppressor the same loads meter about 148dB.)

Is Barrel Length Still a Factor with Suppressors?

Yes. Even with low-pressure .22LR ammunition we can see something interesting: Barrel length has a significant effect on muzzle report. Shooting the same loads through a rifle and a pistol (barrel length with AAC Element II suppressor 9″ vs. 25″ for the rifle) the muzzle report is about 6dB higher from the pistol.*

I also ran a variety of subsonic 300BLK loads through two different AR-15s using the same suppressor (an AAC Cyclone): one gun with an 8″ barrel, the other with a 16″ barrel. The shorter barrel produced peak sound pressures 3-7dB higher than the longer barrel (depending on the powder load, as we will see below).

Are Suppressors Effective on Autoloading Actions?

Yes. A common question is whether a suppressed autoloading (i.e., semi-automatic) gun is louder than one with a locked action. Modern autoloaders use gas pressure and/or momentum from the discharged round to eject the empty case and load a new one. This usually occurs while the bore still contains a significant amount of propellant pressure. I.e., some of the same propellant that produces the muzzle report comes out of the breech.

Once we add a suppressor can the breech report exceed the muzzle report? It turns out that it can if a gun is poorly tuned, but that with typical guns and loads designed for them it does not. For example, I tested both a .22LR pistol (the Buckmark) and 10/22 rifle (Feddersen-barreled Ruger). Whether I let the actions cycle normally or held the bolts closed, the peak sound levels were the same, with one exception: 60gr Aguila ammo – which is very exceptional ammunition: Its bullet is 50% heavier than almost any other .22LR, and it is loaded in a .22 Short case. As a result, the case unplugs the breech before the bullet even leaves the muzzle! Since there is no suppressor on the breech, on the rifle this releases pressure of 128dB (vs 121dB from the muzzle with the breech locked) and on the pistol it produces 130dB (vs 127dB from the muzzle with the breech locked). However, it is possible to tune these guns to this unusual round by using heavier bolts and/or springs to prevent the action from unlocking before the bullet has left the barrel.

Similar mismatches can be produced with other actions. In fact, just attaching a suppressor to a centerfire autoloader that wasn’t designed for one can be such a nuisance that many designs and components now allow for the gas system to be adjusted. But as another test: I ran a wide range of subsonic loads through my 300BLK AR-15 with its standard gas system in place (i.e., autoloading), and then with its gas port completely blocked (i.e., locked breech). The peak sound pressure levels were identical in each scenario.

Does Powder Load Make a Difference?

Yes. The standard subsonic 300BLK load uses a 10.4gr charge of a relatively slow powder in order to provide enough gas volume to cycle a wide range of guns. I have experimented with other loads to see how light a powder charge I can use while still cycling my guns and producing the same muzzle velocity (about 1000fps) with the same 220gr bullet. (The only way to do this is to use faster burning powders.) I hypothesized that lighter charges would also reduce noise. Testing with the LxT1 sound pressure meter confirms this:

Powder Charge

8″ barrel dB

16″ barrel dB

A1680 10.4gr

140

137

IMR4227 9.0gr

138

132

Steel 7.7gr

137

130

*In fact, to first order we can look at a suppressor as a more voluminous bore, which allows propellant pressure to drop before it comes out the muzzle. The 4.5″ barrel on the pistol has a bore volume of 3cc, while the 16″ barrel on the rifle has a volume of 10cc. The suppressor used here in testing the .22LR firearms (an AAC Element II) has a free volume of 41cc – i.e., the same volume as a 5-foot long barrel! (In practice, of course, suppressors also have baffles which introduce turbulence and other dynamics that further slow the flow of propellant out the muzzle. These features produce an even quieter muzzle report than would be expected based on volume alone.)